3,5-Dimethyl and 3,5-Di-tert-butylpyrazolato Complexes with Alkali Metals: Monomeric, Dimeric, Cluster, and 1D Chain Structures

Ion-bearing stairs: alkali metal complexes of 1,2-diaza-4-phospholides

In this work, eight alkali metal complexes with 1,2-diaza-4-phospholide ligands were prepared and characterized by X-ray single-crystal structural analysis and NMR spectroscopy. Their structures showed varied coordination motifs: (i) a dimeric 1,2-diaza-4-phospholide lithium complex with exo-bidentate bridging coordination (4) consists of two lithium atoms that are linked via two μ₂-bridging, κN,κN'-coordinated ligands; (ii) the polymeric chain 1,2-diaza-4-phospholide potassium complex (5) showed an ion-bearing stair-shaped chain structure running through axis a, where the steps are η² interactions, and there is a transition platform between every two stairs; (iii) the polymeric chain 1,2-diaza-4-phospholide potassium complex (6) also presented a polymeric chain structure in the solid state but displayed a head-to-tail arrangement of two 1,2-diaza-4-phospholides; (iv) in comparison to 6, the 1,2-diaza-4-phospholide sodium complex (7) displayed a tetrameric structure, in which the sodium ions are arranged in a distorted tetrahedral fashion and each of them occupies a vertex of the tetrahedron; (v) the polymeric chain 1,2-diaza-4-phospholide potassium complex (8) presented a solvent-free chain structure, in which potassium ions each is η⁵-bonded by two 1,2-diaza-4-phospholides and η²-coordinated by another, consisting of a stair-shaped chain structure running through axis a but without significant intermolecular contacts between the adjacent stairs in comparison to that of 5; (vi) the polymeric chain 1,2-diaza-4-phospholide sodium complex (9) presented a solvent-free chain structure, in which sodium ions each is η¹(N),η²(N,N),η¹(P)-bonded by three 1,2-diaza-4-phospholides, consisting of a chain structure running through axis a; and (vii) the treatment complex 8 with elemental sulphur or selenium in the presence of crown ether gave rare thiophosphonato potassium [η³(S,P,S)-3,5-tBu₂dp-(μ-K)(S₂)([18]crown-6)] (10) or a selenophosphonato potassium [η³(Se,P,Se)-3,5-tBu₂dp-(μ-K)(Se₂)([18]crown-6)] (11). Both of the complexes crystallized in the orthorhombic space group Pnma as pale-yellow (or red) crystals. The X-ray diffraction analysis revealed 10 or 11 as a terminal complex with the η¹,η¹-X,X-coordination mode (X = S and Se). The ¹H DOSY NMR spectroscopy study of the species 8 in DMSO-d₆ suggested that polymeric complexes (4-9) in the solid state should dissociate into the related monomers in the solutions when the donor solvents were used.

Synthesis of aminyl biradicals by base-induced Csp3–Csp3 coupling of cationic azo dyes

Deprotonation of cationic azo dyes results in the formation of aminyl biradicals.

The synthesis of the industrially important polymer parylene is achieved by polymerization of p-quinodimethane (p-QDM). The polymerization is thought to proceed via a biradical p-QDM dimer, but isolation or characterization of such a biradical has remained elusive. Here, we describe the synthesis of an aza-analogue of this p-QDM dimer. The biradical is formed by base-induced dimerization of an azoimidazolium dye. Due to the presence of sterically shielded aminyl radicals instead of terminal H₂Ċ groups, the stability of this dimer is sufficient for analyses by ESR spectroscopy and X-ray crystallography. A similar Csp³–Csp³ coupling was observed for an azotriazolium dye, suggesting that base-induced C–C coupling reactions can be realized for different types of azo dyes.

A pyrazolate-stabilized sodium hydride complex

The synthesis and structural characterisation of the first sodium hydride complex stabilised by sterically demanding pyrazolate ligands are presented.

Neutral alkaline-metal and alkaline-earth-metal derivatives of imidazole and benzimidazole

A theoretical study of the minima and connecting transition states of the neutral complexes formed by alkaline-metal and alkaline-earth-metal derivatives of imidazolate and benzimidazolate anions has been carried out using B3LYP/6-31+G(d,p), B3LYP/6-311+G(3df,2p), and G3B3 methods. Two and three nondegenerated minima and two and four TS structures have been identified for imidazole and benzimidazole derivatives, respectively. The most stable minima of the alkaline-metal derivatives of both systems correspond to the metal interacting with the imidazole ring, whereas in the alkaline-earth-metal derivatives, the preferred minima depend on the substituent. A remarkable feature of some minima is the fact that some of the metal-aromatic interactions follow the classical π-cation pattern, even though the global structure corresponds to a neutral salt, constituting a class of noncovalent interaction of great interest in the chemistry of aromatic and heterocyclic complexes. A CSD search has confirmed that the two bonding modes, N-σ and π, are present in the solid phase. The π mode has been analyzed by comparison with other azoles.

Novel dinuclear dimethylamido-3,5-dimethylpyrazolato and tetranuclear dimethylamido-3,5-dimethylpyrazolato-polyoxo zirconium(IV) complexes. Synthesis and structural characterisation

The dinuclear dimethylamido-tris(3,5-dimethylpyrazolato)-zirconium(IV) complex [Zr(3,5-Me2Pz)3(NMe2)]2 1 is prepared by treatment of [Zr(NMe2)4] with 3 equivalents of 3,5-dimethylpyrazole (3,5-Me2PzH) with elimination of dimethylamine. When [Zr(NMe2)4] reacted with 2 equivalents of 3,5-Me2PzH, the bis(dimethylamido)-bis(3,5-dimethylpyrazolato)zirconium(IV) compound [Zr(3,5-Me2Pz)2(NMe2)2]2 2 is obtained. Hydrolysis of [Zr(3,5-Me2Pz)3(NMe2)]2 in wet toluene affords the tetranuclear oxo compound [Zr4(eta2-3,5-Me2Pz)4(NMe2)2(mu3-O)2(mu2-3,5-Me2Pz)4(mu2-NMe2)2] . All synthesised compounds are characterised by NMR spectroscopic and analytical methods. Single crystal X-ray diffraction analysis has established the molecular structures of 1 and 4.

Pyrazoles and pyrazolides-flexible synthons in self-assembly

This Perspective summarises the chemistry of the pyrazole ring, and reviews the metal coordination modes adopted by 1H-pyrazoles and their anions. Pyrazolide anions are probably the most versatile ligands in coordination chemistry, with 20 different terminal or bridging coordination modes having been identified so far. Metal cluster compounds supported by pyrazolido ligation are surveyed, concentrating on those reported during the past ten years. Highlights include the wide structural diversity in apparently simple main group pyrazolides; luminescence and charge-transfer complexes in coinage metal pyrazolide clusters; the use of robust metal pyrazolide clusters to construct liquid crystals, supramolecular materials and metal-organic frameworks; and supramolecular complexes formed by pyrazolide-supported metallacrowns.

The structure of alkali metal derivatives of azoles: N-sigma versus pi structures

High level ab initio calculations have been used to study the relative stability of N-sigma and pi configurations of the neutral alkaline derivatives of azoles. The N-sigma structure is the one normally expected for nonionized azolate salts. However, the results show that in the case of the pyrrole and imidazole the pi configuration is more stable than the N-sigma one. The preference of the N-sigma vs pi configurations is related to the presence or the absence of two contiguous nitrogen atoms in the azole ring. A search in the CSD shows that some pyrrolate and imidazolate salts exist in solid phase in the pi configuration.

Alkali-Metal Pyrazolate Complexes with Unusual Pyrazolate Coordination Modes and Pseudocubane Motifs

The dimeric complex [Li(Ph2pz)(OEt2)]2 (1) and tetrameric cluster [Na(Ph2pz)(thf)]4 (2) were prepared by treatment of alkali-metal reagents (nBuLi and Na{N(SiMe3)2}, respectively) with 3,5-diphenylpyrazole (Ph2pzH) in Et2O (1) or THF (2). The polymer [Na(tBu2pz)]n (3) was obtained from reaction at elevated temperature in a sealed tube between Na metal and 3,5-di-tert-butylpyrazole (tBu2pzH). The complex [Na4(tBu2pz)2(thf)3(obds)]2 (4; obds = (OSiMe2)2O) was obtained as a minor product from prolonged treatment of tBu2pzH with elemental sodium in a silicone-greased flask. All four alkali-metal pyrazolato complexes were characterized by IR and 1H NMR spectroscopy and X-ray crystallography.The Li dimer 1 displays mu-eta(2):eta(1) lithium-pyrazolato binding, in which both lithium atoms are four-coordinate. Room- and variable-temperature NMR studies (1H, 13C, and 7Li) of 1 suggest similar behavior in solution, with peaks coalescing at low temperatures. Complexes 2 and 4 display distorted cubane structures. In 2, all the sodium atoms are five-coordinate, whereas 4 contains two sodium/pyrazolate/thf clusters (4:2:3 ratio) bridged by two obds(2-) units, as well as two four-coordinate and two five-coordinate sodium atoms. Compound 3 is composed of two independent chains with the unusual coordination modes mu3-eta(5):eta(2):eta(2), mu3-eta(5):eta(2):eta(1), and mu3-eta(4):eta(2):eta(1), with five-, six-, and seven-coordinate sodium atoms. Two oxo-centered M8 cage complexes [(tBu2pz)6Li8O] (5) and [(tBu2pz)6Na8O] (6) were obtained as by-products from attempted preparation of [Li(tBu2pz)] and [Na(tBu2pz)], respectively, and their structures were determined.

Experimental and theoretical study of the coordination of 1,2,4-triazolato, tetrazolato, and pentazolato ligands to the [K(18-crown-6)]+ fragment

Treatment of 3,5-diisopropyltriazole, 3,5-diphenyltriazole, 3,5-di-3-pyridyltriazole, phenyltetrazole, pyrrolidinyltetrazole, or tert-butyltetrazole with equimolar quantities of potassium hydride and 18-crown-6 in tetrahydrofuran at ambient temperature led to slow hydrogen evolution and formation of (3,5-diisopropyl-1,2,4-triazolato)(18-crown-6)potassium (88%), (3,5-diphenyl-1,2,4-triazolato)(tetrahydrofuran)(18-crown-6)potassium (87%), (3,5-di-3-pyridyl-1,2,4-triazolato)(18-crown-6)potassium (81%), (phenyltetrazolato)(18-crown-6)potassium (94%), (pyrrolidinyltetrazolato)(18-crown-6)potassium (90%), and (tert-butyltetrazolato)(18-crown-6)potassium (94%) as colorless crystalline solids. (1,2,4-Triazolato)(18-crown-6)potassium was isolated as a hemi-hydrate in 81% yield upon treatment of 1,2,4-triazole with potassium metal in tetrahydrofuran. The X-ray crystal structures of these new complexes were determined, and the solid-state structures consist of the nitrogen heterocycles bonded to the (18-crown-6)potassium cationic fragments with eta2-bonding interactions. In addition, (3,5-diphenyl-1,2,4-triazolato)(tetrahydrofuran)(18-crown-6)potassium has one coordinated tetrahydrofuran ligand on the same face as the 3,5-diphenyl-1,2,4-triazolato ligand, while (3,5-di-3-pyridyl-1,2,4-triazolato)(18-crown-6)potassium forms a polymeric solid through coordination of the distal 3-pyridyl nitrogen atoms to the potassium ion on the face opposite the 1,2,4-triazolato ligand. The solid-state structures of the new complexes show variable asymmetry in the potassium-nitrogen distances within the eta2-interactions and also show variable bending of the heterocyclic C2N3 and CN4 cores toward the best plane of the 18-crown-6 ligand oxygen atoms. Molecular orbital and natural bond order calculations were carried out at the B3LYP/6-311G(d,p) level of theory on the model complex, (phenyltetrazolato)(18-crown-6)potassium, and demonstrate that the asymmetric potassium-nitrogen distances and bending of the CN4 core toward the 18-crown-6 ligand are due to hydrogen bond-like interactions between filled nitrogen-based orbitals and carbon-hydrogen sigma orbitals on the 18-crown-6 ligands. Calculations carried out on the model pentazolato complex (pentazolato)(18-crown-6)potassium predict a structure in which the pentazolato ligand N5 core is bent by 45 degrees toward the best plane of the 18-crown-6 oxygen atoms. Such bending is induced by the formation of intramolecular nitrogen-hydrogen-carbon hydrogen bonds. Examination of the solid-state structures of the new complexes reveals many intramolecular and intermolecular nitrogen-hydrogen distances of < or =3.0 A which support the presence of nitrogen-hydrogen-carbon hydrogen bonds.